1. Field of the Invention (Technical Field)
The present invention is related generally to methods and apparatuses for measuring the position, velocity, energy and impact characteristics of a projectile traveling at supersonic speeds.
2. Background Art
Existing methods for the measurement of the velocity and intersection position of a shot on a target are based on pressure shockwave concepts. A supersonic projectile generates a conically shaped expanding shockwave called a Mach-cone where the local and temporal air turbulence effects can be characterized by a steep change in ambient air pressure (shock front) which expands radially outward from the projectile path. However, existing systems are based on directional triangulation techniques where an acoustical sensor array is used only to determine the acoustic energy emitted by a passing bullet and determines the directions from which this shock front arrives at the sensor microphones.
Primary examples of the current prior include U.S. Pat. No. 5,349,853 (Oehler) and U.S. Pat. No. 5,025,424 (Rohrbaugh). Both employ the acoustical energy directional technique, but address the issue of accuracy with two divergent approaches. Oehler uses a complete ballistic history computation procedure to improve measurement resolution and Rohrbaugh uses a sensor design to meet the same goal.
Oehler's invention is designed to observe the complete ballistic history of a projectile with acoustical shock wave mapping being only one component. Oehler does employ acoustical sensors, but uses a data-acquisition design, measurement and positional computational scheme based on full ballistic profiling. Oehler employs the integration of three separate measurements to predict the projectile impact point. These are (1) a firearm-pressure-strain measurement of bullet explosive characteristics and drive-force pressure, (2) a series of bullet trajectory (muzzle) detector measurements to determine initial projectile path and velocity parameters, and (3) three-point acoustical sensory array measurements to determine time-of-arrival and relative spacial displacement. This information is then integrated by an external personal computer program to compute the full trajectory profile of the bullet from the muzzle to the target. Oehler thus requires multiple measurement procedures and instrumentation for bullet placement determination, velocity, trajectory and relative time measurements. At the target plane, Oehler uses three acoustical sensors in a triangular format, for common time-zero reference determination relative to the time the bullet left the muzzle and nominal spacial positioning for the overall ballistic computation and "hit" location prediction. The three-point system restricts the relative target-area operational field-of-activity. The communication link from the acoustical array is land-line based. This limits functionality, range, and use. Oehler cannot perform any form of self analysis and diagnostic checks. In summary, Oehler is a full-profile ballistic measurement system designed to determine the characteristics of the bullet trajectory from the muzzle to the target. As such it is not designed to be portable or for general use by the public.
U.S. Pat. No. 5,025,424 (Rohrbaugh) discloses an automatic shock wave scoring apparatus for scoring a "hit" of a supersonic projectile. The Rohrbaugh invention is a single-site, fixed-location, automatic gunnery targeting system which uses the shock profile of a passing projectile to determine the placement of the projectile impact point above the sensor field. It employs several curved acoustic sensor rods which are positioned below the target-active area. These curved sensor rods are surface pressure-sensitive (to the acoustical shock wave) such that a secondary transverse shock wave is generated in each sensor by the incident shock cone. These secondary waves then propagate through each sensor to the transducers located at their ends. The relative time difference between the arrival of the secondary shock at each end is then used to determine the point of incidence of the projectile shock point on the outside of each sensor. Each curved sensor effectively emulates a two-dimensional array of discrete sensors with first incidence discrimination. In effect, they act like fan detectors to the passing projectile. Based on the geometry of these fan detectors, curvilinear remappings are projected and the relative position of the incident projectile is computed. In general, Rohrbaugh is designed for target projectile mapping. The basis of Rohrbaugh's invention is the unique form, composition, and action of the discrete fan detectors. Rohrbaugh employs two curved (polar) two-dimensional sensors that sense the profile of the shock in a curvilinear polar space, with all of the sensors positioned below the target area. As such, targeting is based on a two-point detection scheme employing a vector-directional cross-correlation technique. The current invention employs four discrete sensors, each of which samples a single point on the surface of the shock cone, within an orthogonal Cartesian coordinate system to ensure independence and separability in its sensor measurements. The Rohrbaugh unit requires resolution of a 2-dimensional cross-dependent projection remapping to determine positioning. The current invention employs simple orthogonal coordinate transform equations.
Other patents which relate to targeting include U.S. Pat. No. 5,247,488 (Borberg et al.), U.S. Pat. No. 4,885,725 (McCarthy et al.), U.S. Pat. No. 2,925,582 (Mattei et al.), U.S. Pat. No. 4,514,621 and 4,282,453 (Knight et al) and U.S. Pat. No. 4,261,579 (Bowyer et al.).